With funding from AHRQ, we studied adverse drug events among 30,000 older adults treated in the ambulatory setting by a multispecialty group practice over a 1-year period. The overall rate of adverse drug events was 50.1 per 1,000 person-years, with a rate of 13.8 preventable adverse drug events per 1,000 person-years. We identified eight categories of errors in medication prescribing and monitoring that were the proximal causes of serious preventable events. Subsequent to this study, AHRQ supported us through the HS-07-003, Ambulatory Care Patient Safety Proactive Risk Assessment program to examine the underlying processes leading to these errors. Working collaboratively with the Fallon Clinic of Central Massachusetts, we employed socio-technical probabilistic risk assessment techniques to understand failures in the medication handling system. Based on this assessment, we are proposing a multi-pronged intervention to lower the rate of the medication error that was most commonly found to cause serious adverse drug events: inadequate laboratory monitoring for high risk medications. Our analysis of system failures underlying this error revealed a total lack of redundancy in this area of the clinic's system as well as several communication problems. In collaboration with Fallon Clinic staff members, we have designed a multi-pronged intervention to add redundancy to the system and specifically address the most common failures identified in the assessment. The intervention includes computerized alerts about therapeutic monitoring to prescribers, telephone and mailed reminders to patients, alerts of overdue lab tests to prescribers and clinic staff, and inclusion of educational messages in summary information patients receive after a visit. Our goal is to improve patient safety by improving the rate of ordering and completion of laboratory monitoring of high-risk medications in the ambulatory setting. Our aims are to implement the multi-pronged intervention; evaluate its impact on rates of laboratory monitoring; assess any unexpected consequences for patients, prescribers, and clinic staff; estimate the costs associated with the intervention and the potential return on investment; and prepare a toolkit to enable implementation in other ambulatory settings. [unreadable] [unreadable] PUBLIC HEALTH RELEVANCE: We have identified the major errors in prescribing and monitoring medications for outpatients that cause adverse drug events. Working with a large multispecialty group practice, we have examined problems in the clinical system that lead to these errors. We are proposing a multi-component intervention to overcome the problems that were found to lead to the most important error: lack of laboratory test monitoring of patients taking high risk medications. The intervention includes alerts to prescribers recommending laboratory test orders and telephone and mailed reminders to patients. [unreadable] [unreadable] [unreadable] [unreadable]